Yamaha YM2413

OPLL, is an FM synthesis sound chip manufactured by Yamaha Corporation.

It is related to Yamaha's OPL family of FM synthesis chips, and is a cost-reduced version of the YM3812 (OPL2).

Common to the OPL family, it features a percussion mode (or rhythm mode) that turns the last three channels into percussion/rhythm channels to produce 5 percussion/rhythm sounds.

To make the chip cheaper to manufacture, many of the internal registers were removed.

There were also some other cost-cutting modifications: the number of waveforms was reduced to two, additive mode was removed along with the 6-bit carrier volume control (channels instead have 15 levels of volume), and the channels are not mixed using an adder; instead, the chip's built-in DAC uses time-division multiplexing to play short segments of each channel in sequence, which was also done in the YM2612 much later.